1. Field of the Invention
The present invention relates to a thin film layered product which may be used to make electronic heatsinks as submounts for high-power semiconductor lasers and devices.
2. Description of the Related Art
As the demands for integration, high power and high responsibility of electronic devices have increases, it has become important to drain away the heat generated by an element itself quickly to the outside so as to improve the performance of the device. A circuit substrate for electronic parts requires insulation and the technique of forming an electric conductor on the substrate is especially important. On the other hand, a heatsink for high energy output elements such as a power transistor and semiconductor laser needs a different technique of dealing with extremely high heat energy density. Especially, the heat dissipation efficiency of a heatsink is required to be as high as 100 times that of the circuit substrate. Severe electrical and mechanical properties are also required.
In some conventional devices such as a semiconductor laser element and a high frequency device, natural or artificially synthesized diamond excellent in thermal conduction is used as a heatsink (Japanese Unexamined Patent Publication (KOKAI) No. 120979/1975). A conventional heatsink, however, has the following disadvantages. There is a difficulty in processing to make a heatsink by thinning single crystal diamond; there is a limitation in the area to be used which depends on the size of the original single crystal diamond; and it is expensive.
It has also been proposed to apply to a heatsink a diamond thin film synthesized by CVD method under the low pressure as a self-standing polycrystalline film or one deposited on an appropriate material having high thermal conductivity (Japanese Unexamined Patent Publication (KOKAI) No. 146793/1986 and Japanese Unexamined Patent Publication (KOKAI) No. 245463/1992). These prior arts, however, have the following disadvantages. Compared with a coefficient of thermal expansion of a metal used for a submount or silicon used generally as a device material, a coefficient of thermal expansion of diamond is extremely small, which causes thermal deformation of the heatsink. Handling is difficult because a diamond polycrystalline film is easily cracked. A heatsink of a single film lacks reliability in insulation because a solder material may exude through a crack. Furthermore, the roughness of a surface of a CVD polycrystalline film reaches 1 to 10 .mu.m and polishing of the surface will be needed depending on purposes, which will raise the cost.
Furthermore,it has been proposed to utilize as a heat conducting substrate a layered product in which CVD polycrystalline diamond films are bonded by organic resin or glass (Japanese Unexamined Patent Publication (KOKAI) No. 333266/1992). This publication also refers to metallic brazing and discloses a composite interlayer filled with diamond particles as adhesives to improve thermal conductivity. For a practical use as a heatsink, however, it has been found that further inventive effort is necessary to improve the insufficient heat dissipation efficiency of the layered member bonded by organic resin or glass, and to ensure enough adhesion between a layered product and an element bonded on one surface of the layered member or a heat dissipating material bonded on the other surface thereof by metallic brazing.